Customer deposits rose by $167.8 million to $853.9 million in the December quarter versus $686.1 million in the September quarter, helping the free cash flow results. Analysts said the improving customer deposit figures are driven by its recently announced Tesla Semi truck and next generation Roadster.

"Investors will rightly question the quality and repeatability of the FCF development, with the improvements in inventory not repeatable, uncertainty around the sustainability of increasing customer deposits and benefits from [Zero Emission Vehicle] credits," analyst George Galliers wrote in a note to clients Thursday.

Tesla also implied that the customer deposit benefit may not occur in the future. The company was asked by an analyst about the drivers for the cash flow improvements in the quarter.

"Some of those are not repeatable. We significantly reduced the finished goods inventory of S and X in Q4, which will not repeat itself going forward. … Customer deposits may not be as well," Tesla"s chief financial officer, Deepak Ahuja, said on the earnings call Wednesday.